Interpretive Summary: Many studies have shown that individual quantitative trait loci (QTL) affecting economic traits can be detected and mapped via linkage to genetic markers. Using DNA microsatellites, QTL of US Holstein bulls were detected via a granddaughter design and matched against a measure of their daughters' milk, fat, and protein yields; fat and protein percentages; productive life; and somatic cell scores. Significant effects of QTL on yield traits were greatest for fat percentage. Results for one highly significant effect on fat percentage were confirmed by a daughter design analysis of the Israeli Holstein population. Detection and confirmation of QTL will allow breeders to make better use of genetic markers when making selection decisions.

Technical Abstract:
DNA microsatellites were used to detect individual loci affecting economically important quantitative traits of dairy cattle via the granddaughter design. Eighteen U.S. Holstein grandsires and 1,555 of their sons were genotyped for 30 genetic markers located on 19 of the 29 bovine autosomes. From 16 to 205 sons were genotyped per family. Of 14,650 son genotypes determined, 77% were informative. The genotype data were matched with the bulls' daughter deviations for milk, fat, and protein yields; fat and protein percentages; productive life; and somatic cell score. The within-family allele effect was significant at P < .01 for TGLA263 on chromosome 3, CSRM60 on chromosome 10, and CSSM66 on chromosome 14. Effects of TGLA263 and CSSM66 were significant for more than one trait, but the effect on fat percentage was greatest for both loci. The effect of CSSM66 on fat percentage was significant at P < 10 -7. The effect of CSSM66 on fat percentage also was significant for the Israeli Holstein population (P < .001), which was analyzed by a daughter design of seven families. Using maximum likelihood, the QTL was found to have a substitution effect of about .28% fat and probably to be located 10 to 20 cM from CSSM66 proximal to the centromere.